Hey Sam,
What frame would you use? If it is a Worksman or Husky (very similar design & dimension, I believe), then a 2 stroke china girl fits best, unless you have deep pockets and are going to have the bottom loop sectioned in. Personally I'd like to see somebody use the Husky frame which I believe costs less than the Worksman frame but is of much the same quality. I'm in the middle of a Worksman build right now, but not technically a board tracker. The engine I'm using is a PK-80 and I'm ordering a pull start kit for it. (Like I said, I'm not going for a strictly board track look, but more in the spirit of an early motorcycle.) If you could find a Schwinn straightbar frame as found on the Panthers, that has a frame design very much in keeping with board trackers, but has a much bigger in frame area for an engine. Even a four stroke can fit, but snugly. That frame would give more options and in my opinion is every bit as well made as the Worksman. So, you need to be asking yourself two questions... what engine and what frame? Also you need to ask yourself what kind of money you will be able to spend. Board trackers can be beautiful and ring all your bells, but they can also be a money pit...
SB